全文获取类型
收费全文 | 5518篇 |
免费 | 438篇 |
国内免费 | 935篇 |
出版年
2024年 | 30篇 |
2023年 | 100篇 |
2022年 | 102篇 |
2021年 | 146篇 |
2020年 | 265篇 |
2019年 | 260篇 |
2018年 | 260篇 |
2017年 | 243篇 |
2016年 | 250篇 |
2015年 | 246篇 |
2014年 | 205篇 |
2013年 | 453篇 |
2012年 | 177篇 |
2011年 | 276篇 |
2010年 | 164篇 |
2009年 | 289篇 |
2008年 | 273篇 |
2007年 | 295篇 |
2006年 | 265篇 |
2005年 | 299篇 |
2004年 | 211篇 |
2003年 | 245篇 |
2002年 | 210篇 |
2001年 | 163篇 |
2000年 | 122篇 |
1999年 | 124篇 |
1998年 | 115篇 |
1997年 | 116篇 |
1996年 | 86篇 |
1995年 | 101篇 |
1994年 | 99篇 |
1993年 | 61篇 |
1992年 | 95篇 |
1991年 | 72篇 |
1990年 | 68篇 |
1989年 | 66篇 |
1988年 | 36篇 |
1987年 | 38篇 |
1986年 | 31篇 |
1985年 | 36篇 |
1984年 | 31篇 |
1983年 | 21篇 |
1982年 | 47篇 |
1981年 | 20篇 |
1980年 | 20篇 |
1979年 | 16篇 |
1978年 | 12篇 |
1977年 | 12篇 |
1976年 | 6篇 |
1973年 | 3篇 |
排序方式: 共有6891条查询结果,搜索用时 15 毫秒
31.
Patterns of zinc tolerance were examined in eightDeschampsia cespitosa (L.)Beauv. populations from normal and zinc-contaminated soils, using solution culture methods. Zinc-tolerant populations have evolved beneath pylons, and their tolerance patterns (degree, variance, heritability) vary. Tolerance is genetically based in all of them. In contrast to the cases of other species previously reported, some within-population differences are considerable, and may be due to both heterogeneity of the soils zinc contents, various gene combinations and gene recombinations between genotypes. One control population contains many tolerant plants, an original result which is discussed. 相似文献
32.
The hemolymph of the adult female Manduca sexta was fractionated by cascade-mode multi-affinity chromatography (CASMAC) on a main-line tandem column chain containing Zn(2+)-TED, T-gel, Ni(2+)-DPA, and phenylsepharose and a side-line column containing Zn(2+)-DPA. The technique separated some of the previously described major hemolymph proteins, and yielded a number of fractions with simple composition. Some of these fractions contained only less abundant proteins of Manduca hemolymph. Thus, it appears that CASMAC would be a very useful fractionation technique for purification and characterization of the minor proteins of insect hemolymph. 相似文献
33.
A. J. M. Stams H. W. G. Booltink I. J. Lutke-Schipholt B. Beemsterboer J. R. W. Woittiez N. van Breemen 《Biogeochemistry》1991,13(3):241-255
To demonstrate the contribution of atmospheric ammonium to soil acidification in acid forest soils, a field study with13N-ammonium as tracer was performed in an oak-birch forest soil. Monitoring and analysis of soil solutions from various depths on the13N-ammonium and15N-nitrate contents, showed that about 54% of the applied15N-ammonium was oxidized to nitrate in the forest floor. Over a period of one year about 20% of the15N remained as organic nitrogen in this layer. The percentage15N enrichment in ammonium and nitrate were in the same range in all the forest floor percolates, indicating that even in extremely acid forest soils (pH < 4) nitrate formation from ammonium can occur. Clearly, atmospheric ammonium can contribute to soil acidification even at low soil pH. 相似文献
34.
Ester Cecere Orestina D. Saracino Margherita Fanelli Antonella Petrocelli 《Journal of applied phycology》1992,4(4):323-327
A survey is reported of the drifting algal community in Mar Piccolo, a polluted basin subject to sewage outlets. The key role was played by a few key species, mainly floridean red algae. 相似文献
35.
Pollution by inorganic and organic sulphur compounds is increasing and, because of the many environmental hazards associated with these compounds (e.g. toxicity, acidification of rain and freshwater, increase of COD, the greenhouse effect), must be taken seriously. There is a wide variety of sulphur oxidizing bacteria available in nature, and these can be used for the effective control of such pollution. The best way to break the sulphur cycle is to stop it at sulphur which, being insoluble, can be easily recovered (e.g.SO4
2- S2- S0). (Eco)physiological knowledge about the sulphur oxidizing bacteria has proved very useful in the prediction of the performance of sulphur oxidizing communities in actual wastewater treatment systems. Appropriate reactor design, based on this type of study, is essential if such bacterial communities are to function efficiently, especially when toxic sulphides must be treated. This paper reviews the natural and anthropogenic sources of sulphur pollution, its consequences and possible solutions. 相似文献
36.
E. Younang E. Léonard-Stibbe P. Viel M. Defranceschi G. Lécayon J. Delhalle 《Molecular Engineering》1992,1(4):317-332
Quantum chemistry calculations of equilibrium geometry, atomic charges, dipole moment, and frontier orbital energies are carried out on model N-vinyl-2-pyrrolidone, its radical anion and cation, and three protonated derivatives. Attempts to obtain poly(N-vinyl-2-pyrrolidone) films grafted on nickel and platinum electrodes, respectively by cathodic and anodic polarizations, are reported. A thin, covering and insulating film is obtained on the Pt anode, as indicated by surface characterizations carried out with UPS and IRRAS spectroscopies. The fact that the film remains adherent to the metallic surface in spite of a permanent contact with an acetonitrile solution in which poly(N-vinyl-2-pyrrolidone) is soluble suggests that chemisorption has indeed been achieved. 相似文献
37.
N. Bouchriti A. El Marrakchi A. Fahim S. M. Goyal 《World journal of microbiology & biotechnology》1992,8(5):546-549
A 3-year bacteriological survey of an oyster-growing area in Morocco, where the Japanese oyster (Crassostrea gigas) is grown, showed that the contamination of this lagunar ecosystem was not continuous but intermittent and that animal manure and human recreational activities were important sources of pollution. The major source of contamination was of animal origin, except during the summer, when human contamination prevailed. 相似文献
38.
Effects of DNA binding and metal substitution on the dynamics of the GAL4 DNA-binding domain as studied by amide proton exchange. 总被引:1,自引:1,他引:0
下载免费PDF全文
![点击此处可从《Protein science : a publication of the Protein Society》网站下载免费的PDF全文](/ch/ext_images/free.gif)
T. Mau J. D. Baleja G. Wagner 《Protein science : a publication of the Protein Society》1992,1(11):1403-1412
Backbone amide proton exchange rates in the DNA-binding domain of GAL4 have been determined using 1H-15N heteronuclear correlation NMR spectroscopy. Three forms of the protein were studied-the native Zn-containing protein, the Cd-substituted protein, and a Zn-GAL4/DNA complex. Exchange rates in the Zn-containing protein are significantly slower than in the Cd-substituted protein. This shows that Cd-substituted GAL4 is destabilized relative to the native Zn-containing protein. Upon DNA binding, global retardation of amide proton exchange with solvent was observed, indicating that internal fluctuations of the DNA-recognition module are significantly reduced by the presence of DNA. In all forms of the protein, the internal dyad symmetry of the DNA-recognition module of GAL4 is reflected by the backbone amide proton exchange rates. 相似文献
39.
Jeffrey A. McNeely 《Biodiversity and Conservation》1992,1(1):2-18
Humans are making increasing demands on natural ecosystems. One recent study has concluded that our species is consuming or diverting some 40% of the net photosynthetic productivity of our planet. Many habitats are being converted to simpler systems which provide more harvestable goods to people. As a result, genetic diversity, species and whole ecosystems are disappearing; some scientists suggest that as many as 25% of the world's species could be lost in the next several decades.The sinking ark is usually characterized in terms of pollution, habitat loss, poaching, introduced species and illegal trade in wildlife products, but these are symptoms rather than causes. At a more fundamental level, many of the same factors which have enabled pollution to become such a problem have also been responsible for the loss in biodiversity; the most important factor is that the effects of pollutants on biodiversity have been considered an externality, an unintended side effect of industrial activity which brought measurable benefits to people. Development activities which have depleted biodiversity have proven profitable only because the real costs have been hidden.Keeping the ark afloat will require the Five-I Approach: investigation (learning how natural systems function); information (ensuring that the facts are available to inform decisions); incentives (using economic tools to help conserve biodiversity); integration (promoting a cross-sectoral approach to conserving biodiversity); and international support (building productive collaboration for conserving biodiversity). 相似文献
40.
In liver homogenate the biosynthesis ofN-acetylneuraminic acid usingN-acetylglucosamine as precursor can be followed stepwise by applying different chromatographic procedures. In this cell-free system 16 metal ions (Zn2+, Mn2+, La3+, Co2+, Cu2+, Hg2+, VO
3
–
, Pb2+, Ce3+, Cd2+, Fe2+, Fe3+, Al3+, Sn2+, Cs+ and Li+) and the selenium compounds, selenium(IV) oxide and sodium selenite, have been checked with respect to their ability to influence a single or possible several steps of the biosynthesis ofN-acetylneuraminic acid. It could be shown that the following enzymes are sensitive to these metal ions (usually applied at a concentration of 1 mmoll–1):N-acetylglucosamine kinase (inhibited by Zn2+ and vandate), UDP-N-acetylglucosamine-2-epimerase (inhibited by zn2+, Co2+, Cu2+, Hg2+, VO
3
–
, Pb2+, Cd2+, Fe3+, Cs+, Li+, selenium(IV) oxide and selenite), andN-acetylmannosamine kinase (inhibited by Zn2+, Cu2+, Cd2+, and Co2+). Dose dependent measurements have shown that Zn2+, Cu2+ and selenite are more efficient inhibitors of UDP-N-acetylglucosamine-2-epimerase than vanadate. As for theN-acetylmannosamine kinase inhibition, a decreasing inhibitory effect exists in the following order Zn2+, Cd2+, Co2+ and Cu2+. In contrast, La3+, Al3+ and Mn2+ (1 mmoll–1) did not interfere with the biosynthesis ofN-acetylneuraminic acid. Thus, the conclusion that the inhibitory effect of the metal ions investigated cannot be regarded as simply unspecific is justified.Dedicated to Professor Theodor Günther on the occasion of his 60th birthday 相似文献